In a production process of a semiconductor element, an original pattern in which a circuit pattern is formed, that is, a mask or a reticle (hereinafter collectively referred to as a mask) is exposed and transferred onto a wafer by a reduction projection exposure apparatus called a stepper or a scanner. Since production of a Large Scale Integration (LSI) requires a large manufacturing cost, it is crucial to improve the production yield.
A defect of a mask pattern can be cited as a large factor of degradation in the production yield of a semiconductor element. Accordingly, it is important to detect a defect in an inspection process during a mask production process. Further, it is also important to measure a line width (CD) of a pattern formed in a mask, generate a map of a distribution of a difference value (line width error: ΔCD) between the measured value of the line width and the design value of the pattern, and feed back the map to the mask production process.
In the inspection process of the mask, the light from a light source irradiates the mask through an optical unit. The mask is positioned on a stage, and then scanned by light as the mask is moved on the stage. The light reflected by the mask is incident to a sensor through lenses. The inspection for detecting a defect of the mask is then performed based on the optical image data acquired by the sensor.
A laser light source, wherein the emission wavelength is in an ultraviolet region, or a plasma light source excited by a laser light source can be used as the light source of the mask inspection apparatus. Most of these light sources are pulse light sources. On the other hand, a TDI (Time Delay Integration) sensor can be used as the sensor that acquires the optical image of the mask. The TDI sensor can input optical images at high speed and is therefore an appropriate sensor for the mask inspection apparatus if the sensitivity of the TDI sensor, at the ultraviolet region, is adequate.
When a light quantity of the light source is fluctuated, a gradation value of the optical image data is fluctuated, and it is then impossible to accurately measure a line width of a pattern, and impossible to obtain an accurate CD map. Therefore, the data output from the TDI sensor is corrected based on the measurement result by measuring the light quantity of the light source. Furthermore, in order to acquire optical image data having a high resolution, it is necessary to synchronize a moving speed of the stage to timing for acquiring the image on the TDI sensor. Japanese Patent Publication 2007-93317 discloses an inspection apparatus comprising a light quantity detecting device for receiving light from the mask, adjacent to the TDI sensor, thereby acquiring information regarding the light quantity fluctuation of the light source, correcting the detection sensitivity of the TDI sensor from the information, and acquiring information of the speed fluctuation of the stage.
In the conventional method, a photodiode is used as the sensor for detecting a light quantity fluctuation of the light source. However, there is a problem in that the correction of the output fluctuation of the TDI sensor due to the light quantity fluctuation is not adequate, and a correction error occurs because of the difference between a band of a response frequency of the photodiode and a band of a response frequency of the TDI sensor. The present invention has been made in view of the above-mentioned problems. That is, an object of the present invention is to provide a mask inspection apparatus and a mask inspection method that can obtain an accurate line width by decreasing the above-mentioned correction error.